Automatic combustion control systems and methods are disclosed in my prior U.S. Pat. Nos. 3,861,855; 3,973,898; and 4,043,743. The present invention relates to improvements thereupon and to systems which may use optical pyrometers such as, for example, disclosed in U.S. Pat. No. 3,795,918, which was granted to J. Sunderland on Mar. 5, 1974.
U.S. Pat. No. 3,973,898 discloses a combustion control effected by the use of a smoke detector and by solid-state devices for the detection and amplification of electrical signals produced by a photoelectric cell for the purpose of controlling a motor which in turn adjusts fuel and air supplies. The electrical circuits disclosed therein include triac switches effective for running a servo-mechanism or motor in one direction or the other according to the density of smoke in a flue venting combustion products from a combustion chamber. Light-emitting diodes are connected to amplifiers for the purpose of indicating the type of control being effected. A smoke-accumulating means is employed for amplifying the products of combustion to facilitate the detecting and measuring of the same.
U.S. Pat. No. 4,043,743 discloses a combustion control system based on the detection of smoke density as in the abovedescribed patent. A photoelectric device measures the intensity of light directed through the smoke by a lamp in order to measure the smoke density. This measurement is employed to control fuel-to-air ratio or the like. To supply a standard reference, light is directed from the lamp through an alternate path not affected by the smoke to the photoelectric device. A shutter mechanism alternately exposes the photoelectric device to the light passing through or circumventing the smoke. The photoelectric measurement is compared with a standard and the comparison is used to adjust circuitry connected to the photoelectric device to stabilize the effects of the latter.
In my previous platents, optical sensors measure the solid particulates in flue gas generated by a combustion process. The highest efficiency combustion, i.e., the highest CO.sub.2, occurs just at the visible smoke level, well known as the "light gray haze." The previous patents described how that condition was measured and maintained.
It is well known that the color of an oil-fueled fire is related to completeness of combustion and, hence, to the combustion efficiency. By relating the color to the efficiency as measured by well-known methods such as the Bacharach CO.sub.2 tester and then maintaining that color at all times, the highest possible efficiency for that particular combustion equipment and process can be maintained regardless of the constant changes in the varying parameters of the combustion process, such as air temperature and density, fuel temperature, density, viscosity, air/fuel ratio, etc.
The particulate detection method described in the abovementioned patents, while very effective in many cases, has the following shortcomings:
1. The inherent time delay in response to changes in the combustion conditions in the firebox, due to the remote siting of the sensor, in some cases does not permit fast enough correction to prevent short periods, a few seconds, of visible smoke at the stackhead. This is mostly true of steam and air atomizing types of burners, but less in rotary cup types of burners. Such visible smoke is, of course, objectionable from both the efficiency and environmental points of view.
2. The siting of the sensor is critical, even approaching an art. The length of travel of the flue gas in reaching the sensor must be kept as short as possible to avoid the time delay problem. However, the flue gas travel time depends on the physical dimensions of the boiler and are beyond the control of the installer of the sensor.
3. Considerable clearance, about twenty inches minimum, is required on each side of the boiler to permit mounting of the two sensor boxes. This space is sometimes not available.
4. As the sensor must be able to "see" a true sample of the flue gas for every firing condition, this places an additional difficult criterion on the siting of the sensor.
5. In order for this type of sensor to be able to operate at all, some particulate matter must be present in the flue gas in order to permit the sensor to differentiate between the various operating criteria. Thus, it is sometimes difficult to meet the very low levels of particulate matter prescribed in some environmental statutes.
A discussion of the Sunderland U.S. Pat. No. 3,795,918 will follow later.